ENERGY CHANGES INVOLVED IN SECRETION 265 



It is only for the first of these types of energy production by 

 the cell that accurate quantitative estimations can be made ; 

 because for the second type the chemical energy and amounts 

 of the organic substances formed in the cell, and the chemical 

 energy and amounts of substances used by the cell in their forma- 

 tion, are at present unknown to us. 



Method of Estimating the Work done against Osmotic Pressure in 

 separating each Constituent of a Secretion. The amount of work 

 done in separating each constituent of a secretion can easily be 

 deduced when the pressure of the substance in the lymph and 

 in the secretion are known, and the total volume of the secretion. 

 But such estimation must be made for each constituent of the 

 secretion separately, and the total work done is the sum of the 

 work done in the separation of each constituent. It leads to 

 quite a fallacious result to take the two depressions of freezing-point 

 of the lymph and secretion respectively, calculate the total osmotic 

 pressure of lymph and secretion from these two values, and then 

 assume that the work done is the product of the volume of the 

 solution and the difference in pressure. For the amount of volume- 

 energy change, as has been pointed out in a previous chapter, 

 depends upon the two pressures for each constituent between which 

 pressure has varied for that particular constituent, and since in the 

 formation of a secretion the same ratio is not preserved between 

 the pressures of the various constituents as exists in the lymph, but 

 one constituent is far more compressed or concentrated than another, 

 it cannot be taken that the lymph is compressed or concentrated 

 as a whole as it were by a piston impermeable to all the dissolved 

 constituents, and the work done obtained from the total initial and 

 final osmotic pressures and the change in volume, but instead the 

 work done upon each pressure-giving constituent must be taken 

 separately, and the total work calculated as the sum of all these 

 fractions. 



As demonstrated in a previous chapter, the work done when 

 a grm. molecule of substance is compressed from pressure p l to 



pressure p 2 is given by the expression RT log 2 , and if Q be any 

 other weight in grms. of the substance and M the molecular weight, 



Q 



then the number of grm. molecules will be ^, and the expression 

 for the amount of work done in changing the pressure of the 



